The evaluation of existing concrete infrastructure is needed as design approaches have changed and load demands have increased due to traffic volume increase over the decades. The concrete structures experience increasing crack widths as well as unexpected new cracks especially on bridge structure and railroad ties. These cracks may decrease service life and are possible signs of structural deficiencies. This study is focused on two particular structures that experiencing such cracks: (i) inverted-T bent caps and (ii) railroad ties. Concepts for improving the performance of these structures are developed and experimentally validated. For inverted-T bent caps, to address the ledge flexure, hanger, and/or punching shear deficiencies, which are identified in previous studies based on the current design specification, a conceptual design of eighteen alternative retrofit solutions are developed. The solutions include both metallic and fiber reinforced polymer materials, which are applied either with or without prestress. To identify which retrofit solutions are most viable for further investigation, a weighted sum decision making model is used based on six criteria: (i) strength, (ii) cost, (iii) constructability, (iv) dimensional/clearance constraints, (v) durability/longevity, and (vi) ease of monitoring. Among the most highly ranked solutions, the solutions utilizing prestressing techniques are selected: through-web post-tensioning threadbar with catcher channel (and end-region stiffener) and load balancing post-tensioning solutions. Based on one-half scale experimental tests, results demonstrate that the specimens are strengthened and the solutions are capable of restraining any pre-existing cracks thereby delaying failure. Design and retrofit recommendations are provided, including their limitations. For prestressing concrete railroad ties (sleepers), use of non-metallic aramid fiber reinforced polymer instead of the usual steel strands is explored. Such a switch in prestress materials may help in reducing electric leakage and corrosion issue when in service; however, the strength, stiffness, and crack resistance due to the material change should not be inferior. Test results on sleepers prestressed by aramid fiber reinforced polymer strands demonstrate that the strands are capable of providing sufficient strength and stiffness. Recommendations are made on how the performance could be improved by using smaller diameter strands to delaying further cracking beneath the rail seats.
